Study-Unit-3-lecture-notes-2025

Lecture Unit 3: Photosynthesis

Photosynthesis Overview

• Definition: Photosynthesis is the process by which autotrophic organisms convert light energy into chemical energy, building complex carbon-containing compounds from CO2.

• Nature of Process: An anabolic process, meaning it involves synthesis and the building of molecules.

Autotrophs vs. Heterotrophs

Autotrophs

• Definition: Organisms that can produce their own carbohydrates using only CO2 and water.

• Examples: Plants, algae, cyanobacteria.

• Usage of Carbohydrates: Carbohydrates produced can be directly used or modified into other molecules for both anabolic and catabolic processes.

Heterotrophs

• Definition: Organisms that cannot synthesize their own carbohydrates and must consume other organisms (either autotrophs or other heterotrophs).

• Examples: Animals, fungi, most bacteria.

Photosynthesis vs. Respiration

• Photosynthesis Equation:

  • Sunlight + 6 CO2 + 6 H2O → C6H12O6 + 6 O2

• Respiration Equation (for reference):

  • C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Free Energy.

Key Discoveries about Oxygen Production

• Ruben et al.'s Experiment found that:

  • The oxygen released during photosynthesis originates from water (H2O) and not from carbon dioxide (CO2).

  • Method: Using isotope-labeled water and CO2 produced different labeled oxygen outputs.

  • Conclusion: Water is the source of oxygen atoms released in photosynthesis.

Pathways of Photosynthesis

Light Reactions

• Also known as light-dependent reactions.

• Function: Converts light energy into chemical energy.

• Products: ATP and NADPH.

Carbon-Fixation Reactions

• Also known as light-independent reactions or Calvin Cycle.

• Function: Uses ATP and NADPH along with CO2 to produce carbohydrates.

• Nature of Process: Anabolic, involving the building of molecules.

Structure of Chloroplasts

• Outer and Inner Membrane: Surrounds the chloroplast.

• Stroma: Liquid matrix outside thylakoids where CO2 is converted to carbohydrates.

• Thylakoid Membrane: Site of light absorption by chlorophyll, where light energy is converted into chemical energy.

• Granum: Stacks of thylakoids.

Light Energy and the Electromagnetic Spectrum

• Energy in Light: Inversely proportional to wavelength; shorter wavelengths have higher energy.

• Photons: Particles of light that represent packets of energy.

  • Interactions with Molecules: Can be absorbed, scattered, or transmitted.

Photosynthetic Pigments

• Function of Pigments: Absorb light and play a key role in photosynthesis.

• Chlorophyll: Primary pigment that absorbs light mainly in the blue and red wavelengths; appears green due to reflection of green light.

• Accessory pigments: Broaden the range of absorbed light wavelengths.

Absorption and Action Spectrum

• Absorption Spectrum: Chlorophyll absorbs specific wavelengths and scatters others.

• Action Spectrum: Indicates rate of photosynthetic activity across different wavelengths.

Photosystems in Chloroplasts

• Photosystems: Complexes of proteins and pigments where light energy is captured.

  • Contains light-harvesting complexes that help absorb light.

  • Reaction center where absorbed light energy is converted to chemical energy.

• Energy Transfer: Energy is passed from excited electrons in chlorophyll molecules to chemical acceptors.

Light-Dependent Reactions

• Stages: Light capture, energy transfer, electron transport, and ATP synthesis through chemiosmosis.

• Electron Transport Pathway: H2O → Photosystem II → Photosystem I → NADP+ reductase; produces ATP and NADPH.

Calvin Cycle (Light-Independent Reactions)

• Main Enzyme: Rubisco (Ribulose bisphosphate carboxylase/oxygenase).

• Processes:

  1. Fixation of CO2 with RuBP to create 3PG.

  2. Reduction of 3PG to G3P using ATP and NADPH.

  3. Regeneration of RuBP from G3P.

• Net Gain: For every six cycles, 2 G3P are produced, leading to glucose formation.

Photorespiration

• Mechanism: Occurs when Rubisco reacts with O2 instead of CO2, leading to a loss of carbon efficiency.

• Inhibition of Calvin Cycle: Phosphoglycolate inhibits further Calvin cycle reactions.

• Factors affecting photorespiration: Stoma closure during the day reduces CO2, thus favoring O2 fixation at lower CO2 levels leading to inefficiencies.

Comparisons of Photosynthesis and Respiration

• Differences:

  • Locations: Chloroplasts (photosynthesis) vs. mitochondria (respiration).

  • NADP+ for photosynthesis vs. NAD+ for respiration.

  • Respiration is reversible; photosynthesis is not.

  • Respiration generates net ATP; photosynthesis consumes ATP.

• Similarities:

  • Both processes are energy-converting and involve electron transport chains.

  • Both create and use ATP and have pathways that can interconnect with other metabolic processes.